(1) Field of Invention
The present invention relates to a turbine engine, such as an airplane turboprop or turbojet, the engine being fitted with means for controlling blade tip clearances, and the invention also relates to a method of controlling such clearances.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In conventional manner, air passing through a turbine engine flows from upstream to downstream through a low-pressure compressor and a high-pressure compressor, and then penetrates into a combustion chamber from which the outlet feeds a high-pressure turbine having its rotor driving the rotor of the high-pressure compressor, and then a low-pressure turbine having its rotor driving the rotor of the low-pressure compressor.
In general, the high-pressure turbine comprises a bladed rotor wheel situated between two rows of stator vanes, an upstream row and a downstream row, that are carried by an outer casing, with a small amount of radial clearance being provided between the tips of the moving blades and the outer casing. The rotor wheel comprises a disk carrying the blades and connected to the shaft of the high-pressure turbine.
While the turbine engine is in operation, it is important to minimize the radial clearance at the tips of the blades in order to avoid air leaking and in order to guarantee maximum performance of the engine.
Nevertheless, it is found difficult to adjust this radial clearance, since the dimensional variations in operation of stationary parts are different from the dimensional variations of rotary parts. All of the parts are subjected to temperature variations in the combustion gas, thereby giving rise to successive expansions and contractions depending on the speed of the engine, but the temperature variations and the corresponding dimensional variations of the rotary parts take place more slowly than those of the stationary parts because of the thermal inertia of the mass constituting the rotor disk of the high-pressure turbine. In addition, account must also be taken of the dimensional variations of the turbine blades due to centrifugal forces in operation.
Devices have already been proposed for controlling clearances at blade tips, which devices comprise means for taking air from an upstream portion of the high-pressure compressor, e.g. from its fourth stage, and from a downstream portion of the compressor, e.g. from its ninth stage. Each circuit for taking air includes a valve, with the opening and the closing of the valve being controlled by a control system. The air taken in this way is conveyed to the outer casing in order to cool it or to heat it, thereby adjusting the clearance at the tips of the moving blades of the high-pressure turbine (see document FR 2 828 908-A1 in the name of the Applicant).
The control system receives information relating to the speed of the engine, to the temperature of the outer casing, to the temperature at the outlet from the high-pressure compressor, together with information relating to the operation of the engine (idling on the ground, starting while hot or while cold, temporary acceleration or deceleration, . . . ).
That known device is complex, since it requires separate valves and circuits to be installed for taking air from the upstream and downstream portions of the high-pressure compressor. It is necessary to control the extent to which the valves are opened in order to have full control over the temperature of the air that is to impact against the outer casing, and that likewise is complicated. Furthermore, that type of device is found to be particularly heavy and bulky. Finally, taking air from the downstream portion of the high-pressure compressor is disadvantageous, since it consumes air at very high pressure and thus penalizes the efficiency of the engine.
Another problem arises when restarting the engine while hot, i.e. on restarting the turbine engine after it has been stopped for a length of time that is insufficient for the temperature of the engine and in particular for the temperature of the rotor disk of the high-pressure turbine to have returned to ambient temperature. After the engine has stopped, it is observed that it cools more quickly in its bottom portion (at six o'clock) than in its top portion (at twelve o'clock), which leads to the rotor of the high-pressure turbine occupying an off-center position within the outer casing. Thus, the clearance at the blade tips in the bottom position is reduced and centrifuging the rotor blades of the high-pressure turbine can lead to rubbing against the outer casing in its bottom position.
Proposals have also been made for a device for controlling clearance by electrically heating the outer casing, thus making it possible to accommodate accelerations and to avoid the harmful effects of hot restarts, but that does not enable the casing to be cooled in order to reduce clearance during cruising flight.